I. Introduction to EPR II. EPR in heterogeneous catalysis

I. Introduction to EPR

II. EPR in heterogeneous catalysis

by Song-I HanAssistant ProfessorUniversity of California Santa Barbara

PIRE-ECCI/ICMR Summer ConferenceAugust 14-25University of California Santa Barbara

I. Introduction to EPR

• Basics of cw EPR

• One electron in the magnetic field

• Crystal field splitting and spin orbit coupling

• Interaction with nuclear spins

• Electron nuclear double resonance (ENDOR)

• Electron spin echo envelope modulation (ESEEM)

Nuclear Magnetic ResonanceElectron Paramagnetic Resonance

uses the interaction between the magnetic moments and the magnetic component of electromagnetic radiation in the presence of magnetic fields

spin angular momenta are quantized

magnetic moments through non-zero spin angular momentumof unpaired electrons and a variety of nuclei

Which magnetic field strength and frequency?

NMR: 1 – 21 Tesla and 10 – 900 MHzEPR: 0.06 – 3 Tesla and 2 – 90 GHz

Low energy splitting low Boltzmann polarization

NMR: 1016 spins per 1 mlEPR: 1010-1011 spins per <1ml

Which information?

Atomic, molecular and lattice structure: perturbation of the magnetic moments through the neighboring electronic and nuclear spin network

NMR: nuclear magnetic moments

EPR: electron magnetic moment

• first EPR observation in 1945: CuCl2·H2O (133 MHz at 5 mT)

• rapid exploitation of EPR thereafter through the availability of complete 9.5 GHz (λ=32 mm) systems after World War (II) resonance at 0.35 T field

• application to chemical systems since 1970

• technological availability of magnetic fields and frequency sources limits the common EPR frequency range to 1-100 GHz

• fixed frequency and magnetic field sweep is most common

• only the electronic ground state of species is relevant for EPR

• uniquely applicable to paramagnetic state (net electron angular momentum)

EPR brief

1. free radicals in the solid, liquid or gaseous phase1 unpaired electron

2. transition ions including actinide ionsup to 5 or 7 unpaired electron

3. various point defects (localized imperfections) in solidselectron trapped at a negative-ion vacancy (F center) or an electron hole

4. systems with more than one unpaired electrontriplet state systems, biradicals

5. systems with conducting electronssemiconductors, metals

V4+, Ti3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+ ...

Typical systems studied by EPR

One electron in the magnetic field

g-factor = 2.0023, and nearly isotropic for the simplest case

Η = – µ ·B = γ ·S·B = ge·βe·S·B

negative gyromagnetic ratio γ

Electron Zeeman Interaction (EZI): 2-level system

S = ½, MS = 1/2, -1/2:

|∆MS| = 1, E± = ± ½ g β B

|∆MS| = 1 condition does not hold strictly when S>1/2, or when hyperfine interaction is present

EPR detection: Magnetic field sweep with 100 kHz modulation

Η = ge·βe·Beff·S= βe·B0·g·S

Η = γ·Beff·I= γ·B0·(1-σ)·I

NMREPR

One electron in the magnetic field:2-level systems with resonance around g = 2

1. Free radicals2. Ti(III), 98Mo(V), low-spin Fe(III)3. F centers in alkali halides4. Hydrogen atom trapped in crystal matrices5. Conducting electrons in metals

1H gas, S=1/2, I=1/2 Ge3+, S=1/2, I=9/2

Much more complex EPR spectra due to …

• effects of additional magnetic and electric fields from the unpaired electron’s environment

• presence of more than one electron (e.g. transition metal ions with several unpaired d -electrons, up to 5 for high-spin Mn2+ or Fe3+)

• organic molecules in triplet state

such terms >> electron Zeeman term due to B0IF

• MW frequency may have too small bandwidth (e.g. in VO2+, Mn2+)• EPR spectra may only be observed in the ground state manifold

Crystal-field splitting and spin-orbit coupling

cause large energy splittings in transition metal ions condensed phase

removes the orbital degeneracy for most transition metals“quenching of the orbital angular momentum” effective spin S=1/2

figure: 3d9 transition metal ion

EPR spectra only through the unpaired electron in dx2-y2 orbital

one electron S=1/2, two-level system?

Ligand field effect through second order SOC

molecule’s ground state undergo orbital quenching by chemical bonding (e.g. radicals) or large CF’s (e.g. transition metal ions) g-factor of free electrons

small amount of orbital angular momentum admixes to the ground state through interaction with the excited states

electron spin becomes sensitive to crystalline environment

anisotropic g-factor, i.e. the Zeeman splitting depends on the symmetry of the ligand field and the molecular orientation in B0 Η = g·β·S·B

Most

But

Therefore

Leading to

g tensor!

g anisotropy

EPR spectra depend on orientation of samples in B0

in single crystals or powder samples

Η = ge·βe·Beff·S= βe·B0·g·S

principal values contain symmetry of the inner field⎥

⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

g = gx, gy or gz

B0 along one of the PAS

g2 = gx2 ·lx2 + gy

2 ·ly2 + gz2 ·lz2

arbitrary orientation of B0

CF with axial symmetry

θθθ 22222 cossin)( ggg +=⊥ g|| > g� B0(g||) < B0(g�)

EPR of solutionsMolecular motion leads to simplification of spectra due to averaging of anisotropic interactions

Typical vanadyl solution spectrum (of VO(acac)2)

S=1/2, I=7/2

in order to increase relaxation times through freezing of molecular motionidentical to powder spectra concept of anisotropy is of importance

EPR at low temperature (frozen solids)

S=1/2, I=0

1st derivative

Interaction with nuclear spins(hyperfine interaction)

local magnetic fields by neighboring nuclear magnetic moments (µn)

depend on the orientation of µn with respect to B0

splitting of each electron spin state into (2I+1) levels.

Η = gn·βn·Beff·I= γ·B0·(1-σ)·I

Nuclear Zeeman interaction

NZI << EZIe.g. gn=5.58 for proton

1H, S=1/2, I=1/2 nitroxide radical, S=1/2, I=1

Co(II), V(IV), VO(II)

S=1/2, I=7/2

8 lines

Cobalt (II), 3d7 (single crystal)

Co2+ in Mg(CH3COO)2·4H2O crystal

High-spin manganese (II), S=5/2

S=5/2, 3d5 character in bound form; 5 line EPR spectrumI=5/2, 6-fold splitting of EPR line

Mn2+ in ScPO4 single crystal

Origin of hyperfine interactions

• dipole-dipole interaction between the magnetic moments

⎥⎦⎤

⎢⎣⎡ ⋅⋅−⋅= ))((3

41

230

rgg

hr nnee Sββµπ

Η δδ anisotropic

unpaired electrons in p-, d-, or f- orbitals

• Fermi-contact interaction through finite electron spin density at the nucleus

⋅Ψ= S20 )0(32

nneecontact ggh

ββµΗ isotropic

unpaired electrons in mainly s- or p-,d-. f- orbitals or π electrons or transition metals

The hyperfine interaction matrix A

⋅−⋅⋅+⋅⋅+⋅⋅+⋅⋅⋅+Η=Η nnFS g ββ

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡+=

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡=

δ

δ

δ

isoa1

• isotropic EZI and HFI for I=3/2 (e.g. Cu2+)

• selection rule: ∆mS=±1 and ∆mI=0

• 4 allowed transition

• centered around giso and split by aiso

Powder and single crystal EPR spectraof Cu2+ complexesS=1/2, I=3/2

g|| is larger than g┴

g, A values: copper ion is surrounded by oxygenCu2+ doped into CaCd(CH3COO)4·6H2O

Powder EPR of Cu2+ complexes

S = 1/2, I = 3/2

EZI >> HFI

coaxial and axially symmetricg and A matrices

powder EPR spectrum for an axially elongated copper complex

(a) copper (II) diethyldithiocarbamate(b) copper (II) tetrapyridyl porphyrazine(c) copper (II) in calcium cadmimum acetate hexahydrate

Powder EPR of Cu2+ coordinated to sulfur, nitrogen or oxygen

coordination to close nuclei throughgII and A|| values by CW X-band EPR

ESR observation of the formation of anAu(II) complex in zeolite Y

divalent state of gold is rare andmost interesting oxidation statein transition metal chemistry

zeolites are very promising as supportfor stablizing cations and metalic speciesbecause of their crown-ether-like ringstrcutures in their cages and channels.

first observation of square-planar Au(II)-complex with N4 in the supercage of Y-zeolite was observed via EPR

Z.Qu, L. Giurgiu, E. Roduner, Chem. Commun. (2006) 2507-2509.

Effect of nuclear Zeeman interaction vs. hyperfine interaction at the nucleus

weak coupling:e.g. water protons in the first coordination sphere ofcopper-hexaaquo complex

strong coupling:typical for most systems

The complete Hamiltonian

⋅−⋅⋅+⋅⋅+⋅⋅+⋅⋅⋅=Η nng ββ1. ... so far one electron spin in magnetic fields of different origins were discussed

2. Zero-field splittingmore than one electron spins (S > 1/2) in magnetic fields strong dipole-dipole interactions between the electronse.g. transition metal ions with up to five unpaired d-electrons or spin triplets in organic moleculescan be much larger than the electron Zeeman interaction

3. Nuclear Quadrupole Interactionfrom nuclei with a nuclear spin quantum number I > ½impact on EPR spectrum is usually small

Relaxation times and linewidths

T1 spin-lattice relaxation:through dissipation of energy via the thermal vibration of the lattice

T2 spin-spin relaxation:mutual spin flips caused by dipolar and exchange interactions among the spins

linewidth determined by:12,2 2

111TTT eff

+= (T2 is much shorter than T1)

for transition metal complexes, T2 = several µs at liquid helium temperatures

Sensitivity and resolution

e.g. for a commercial X-band spectrometer 5·1010 spins can be detected at a SNR of 1.5 for a 1 gauss wide line

e.g. linewidths of transition ions in crystals, present at low concentrations,or in ~1mM solutions, are typically 10–100 gauss

A list of paramagnetic ions observed in EPR

no. of d-electrons

55 out of 106 elements belonging to transition-group, rare-earth and actinide ions, that is, the members of the 3d, 4d, 5d, 4f and 5f groups have been subject of EPR investigations.

* Biologically important elements are V, Fe, Mn, Co, Cu, Mo and Ni

Chromium (III), S=3/2 (single crystal)

S=3/2 four electronic energy levels

Guanidine aluminum sulfate hexahydrate (GASH) doped with 1% copper (II)

EZI >> ZFI3 transitions

splitting due to 2 sites crystal

Iron (III)

Low-spin caseS=1/2

High-spin caseS=5/2, 3d5 character in bound form5 line EPR spectrum

Fe3+ in AgCl crystal at 20K

g=2.0156, a=0.0075low spin Fe3+ in cytochrome P450 protein

g=2.42, 2.25, 1.92

High-spin iron (III), S=5/2

EZI >> ZFI EZI << ZFI

Fe3+ in AgCl crystal at 20K, cubic symmetry

g-2.0156, a=0.0075Fe (III) protein, metmyoglobin at 2K

g=6, 2

Electron Nuclear Double resonance (ENDOR)

CW EPR CW ENDOR

EPR saturationNMR saturation

frequency sweep

EPR detection

probe structural detail due to unresolved hyperfine structuresdetermine both strong and weak nuclear hyperfine couplingsand from more distant nuclei

indirect NMR detection by EPR

solid EPR spectra are ofteninhomogeneously broadened

EPR frequency NMR frequency

Electron Spin Echo Envelope Modulation (ESEEM)

indirect NMR detection by EPR

two allowed transitions (∆mS=±1, ∆mI=0)two forbidden transitions (∆mS=±1, ∆mI=±1)

electron-nuclearcoupled system

utilizing frequencies of forbidden transition that contains electron-nuclear hyperfine interaction

echo modulation

ESEEM to the study of bonding between Cu(II) ions and hydrated metal oxide surfaces in the presence of phosphates

Cu(II) (pyrophosphate)2

Cu(II) (pyrophosphate)2 adsorbed on δ-Al2O3 Cu(II) NTP adsorbed on δ-Al2O3

Dynamic Nuclear Polarization

sensitivity enhancement of NMR by EPR

utilizing higher γlow temperatureof electron spins

polarized water

unpolarized water

• cw ENDORstrong hyperfine coupling

insensitive

• ESEEMpeaks at same position as in ENDOR

weak hyperfine coupling (e.g. far nuclei, 14N)

sensitive

only applicable to radicals and transition metal with narrow EPR lines

• 2D HYSCOREweak hyperfine coupling and connectivity to electron spin resonance

only applicable to radicals and transition metal with narrow EPR lines

Nuclear spin coordination around the electron spin through hyperfine coupling

effect increases at lower magnetic fieldX-band is a good compromise

CW EPR and ESEEM are powerful tools for analyzing catalytically important oxide systems containing paramagnetic ions

1. The ESEEM method is particularly well adapted for studying powder spectra in molecular sieves and other oxide solids.

2. Paramagnetic species can be located with respect to various surface or framework atoms via weak electron-nuclear dipolar hyperfine interactions with surrounding magnetic nuclei to distances of about 0.6 nm.

3. By using deuterated or 13C-labeled adsorbates it is possible to determine the number of adsorbed molecules and their distance toa catalytically active center.

CW EPRS-band (2-4 GHz), X-band (8-10 GHz), Q-band (~35 GHz) and W-band (~90 GHz).

reflected MW power as a function of B0 with 100kHz amplitude modulation at constant frequency, <200mW power, high sensitivity

klystron wave guide resonator (rectangular cavity)

Pulsed EPR• small excitation bandwith: ~100MHz through 10 ns pulse (30 Gauss at g=2)• low sensitivity due to low Q/high power/long deadtime ~100ns• low temperature in order to prolong relaxation time• sophisticated technical equipment and theoretical treatment needed• fast acquisition of entire EPR spectrum (a few microseconds)• additional information about weakly coupled nuclei • direct measurements of relaxation rates

II. EPR in heterogeneous catalysis• Defects and radical processes on oxide surfaces

– Color centers on oxides

– Photocatalytic reactions on oxides

– Interfacial coordination chemistry

• Metal centers in microporous materials

• In situ EPR (“operando” EPR)

What can we learn from EPR in heterogeneous catalysis studies?

• explore the nature of active sites

• identify reaction intermediates

• follow the coordination of supported transition metal ions

• follow the oxidation states of supported transition metal ions

• understand electron transfer reactions

Strengths of EPR for heterogeneous catalysis studies

• direct detection of paramagnetic states

• unambiguous identification, low background signal

• high sensitivity (compared to NMR)

• surface and bulk species can be distinguished

• non-invasive technique

in-situ catalytic studies on bulk catalysts materials possible

Colour centers on oxides

the adsorption and catalytic behaviour of simple binary oxides (e.g. MgO) depend on the morphology and defectivity of the oxide itself

the study of localized point detefcts, e.g. anion or cation vacancies andsurface colour centers (FS

+) created by doping anion vacancies with an excess electron is of great interest

EPR is a unique tool:

1. detection of colour centers of <0.02% of the entire surface area

2. location through superhyperfine interaction with surrounding protonsfrom the surface hydroxyls and lattice 25Mg2+ cations

3. detection of radical anion formed through absorption and electron transfer

Reversible formation of N2- radical anion by N2

adsorbtion into surface colour centers of MgO

simulated EPR spectra

surface color center

creation of dinitrogen anion nearly complete electron transfer

E. Giamello, M.C. Paganini, M. Chiesa, D.M. Murphy, J. Phys. Chem. B 104 (2000) 1887-90

at 100K

Finding through EPR, ENDOR and Theory:

defects are formed and stabilized in very low coordinated surface vacancies

the unusual electronic structure of these colour centers, consisting of free electrons in a surface vacancy accounts for their high chemical activity as reducing sites

Photocatalytic reactions on oxides

Motivation:

find detailed information on photogenerated paramagneticspecies in the bulk or surface of photocatalytic materials


for the characterization of photocatalytic materials based on polycrystalline semiconductor oxides, such as TiO2 or ZrO2

1. detect charge carrier states such as trapped electrons or trapped holes on the surface

2. identify the nature of photoproduced radicals

3. monitor their reactivity under various conditions

Photoreduction of CO2 with H2 over ZrO2:

ZrO2+13CO2

+H2, in the dark

ZrO2

color center (F center)

Zr3+

ZrO2+uv

ZrO2

ZrO2+CO2

UV irradiation

CO2- radical anion

13CO2- radical anion (I=1/2)

doublet splitting

the excitation of adsorbed CO2 to form the CO2- radical

seems to be a localized surface process; long-lived speciesY. Kohno, T. Tanaka, T. Funabiki, S. Yoshida, Phys. Chem. Chem. Phys. 2 (2000) 2635-2639.

Transition metal ions dispersed on solid surfaces: role of copper in promoting host metal oxides

Motivation:

to understand synergy between copper promoter and host oxide (e.g. CeO2, SnO2, ZrO2, TiO2) metal ion in NO reduction and CO or hydrocarbon oxidation


to probe the changes in the inner and outer coordination sphere and oxidation state of paramagnetic transition metal oxide or ion during a catalytic reaction

1. structure of the surface complex

2. mechanisms of their interaction with small molecules

Example: Cu(II)/CeO2 catalyst material– coprecipitation from aqueous solutions containing Cu2+ and Ce4+ ions– sorption of Cu2+ ions onto ceria gel

Copper (II) in promoting cerium (IV) oxide catalysts

333K

573K

873K

1073K

1273K

giso~ 2amorphous Cu(II) aggregates on ceria surface(polymeric Cu(OH)2 and hexaaqua {Cu(H2O)6}2+

axial symmetry: g||=2.248, g^=2.092, A||=120G, A^=28Gisolated Cu2+ ions in tetragonally distorted octahedral sites

g||=2.195, g^=2.031, A||=85G, A^=12Gfrom the coupling between two Cu2+ ions (I=3/2)triplet signal from Cu(II) dimer, 7 lines through hsfinterionic distance: 3.1Å

giso=2.098 due to CuO crystallite particles

P.G. Harrison, I.K. Ball, W. Azelee, W. Daniell, D. Goldfarb, Chem. Mater. 12 (2000) 3715-3725.


333K

573K

873K

1073K

1273K

forbidden transition due totriplet states of Cu(II) dimer



isolated monomeric Cu2+ ions are located in siteson the ceria surface similar to those of dimers andare precursors for the dimer formation

333K

573K

873K

1073K

1273K


Exposure of Cu(II)/CeO2 catalysts to COcalcination at 873K, no CO

373K, CO

473K, CO

573K, CO

673K, CO

evacuation at 573K

RT, CO

673K, O2

removal of broad isotropic signaldue to amorphous Cu(II) clusters

removal of Cu(II) dimer signal

removal of monomeric Cu2+ signalappearance of Ce3+ signal

reoxidation at 673K under O2 atm.


Exposure of Cu(II)/CeO2 catalysts to COcalcination at 873K, no CO

373K, CO

473K, CO

573K, CO

673K, CO

evacuation at 573K

RT, CO

673K, O2

Order of reduction

1. amorphous copper(II) dispersed

on the ceria surface

2. dimer species

3. isolated Cu2+ ions

4. CeO2 support (Ce3+ appears)

isolated copper(II) is located withinthe growing crystallites of ceria andits accessibility limited

copper(II) is reduced in preference tocerium(IV) by CO


Exposure of Cu(II)/CeO2 catalysts to COand reoxidation with NO

O2

CONO

RT

323K

373K

isolated Cu2+ ions

Cu(II) ion dimers

Exposure of Cu(II)/CeO2 catalysts to COand reoxidation with NO

NO

RT

323K

373K

isolated Cu2+ ions

Cu(II) ion dimers

lack of Cu-NO signal indicatesthat NO does not bind to Cu sitesbut rather to the ceria surface

Copper (II) in promoting cerium (IV) oxide catalysts in CO oxidation

1. isolated monomeric Cu2+ ions are located in the same siteson the ceria surface as those of the dimers and are precursors

2. dispersed amorphous copper and copper dimers seem most active3. Cu(II) gets more readily reduced than Ce(IV) by CO 4. during reoxidation, NO does not bind to Cu sites but to Ce surface

P.G. Harrison, I. K. Ball, W. Azelee, W. Daniell, D. Goldfarb, Chem. Mater. 12 (2000) 3715-3725.

1. true Ce1-xCuxO2-δ oxide solution was found to be the active phase2. Cu(II) is built into the lattice creating a neighboring oxygen vacancy3. Cu(II) dimers have distances shorter than the Cu-Ce lattice sites

P. Bera et al, Chem. Mater. 14 (2002) 3591-3601.

absorption of a second Cu2+ ion onto the oxygen defect andits reduction (electron transfer) next to the isolated Cu2+ site

hypothesis:

Metal centres in (silico-)aluminophosphate microporous materials1. Aluminophosphate (AlPO-n) and silicoaluminophosphate (SAPO-n) molecular sieves

2. Al and/or P can be replaced by metals to form MeAPO-n or MeAPSO-n materials

3. Me = Ti(III), V(IV), Cr(III), Mn(II), Fe(III), Co(II), Ni(I), Pd(I), Cu(II), Mo(V),

4. The location and structure of the reactive metal ion site and its interaction with different adsorbated and reactants is of importance

5. The incorporation of transition metal ions into framework sites of the ALPO-nand SAPO-n is of particular interest for the design of novel catalysts.

6. A variety of metals can be incorporated into the aluminophosphate structure, but actual incorporation into the tetrahedral framework is difficult to prove.

M. Hartmann, L. Kevan, Chem Rev. 99(3) (1999) 635-663.

Nickel species have been studied intensively in SAPO materials because of their catalytic importance. It is of particular interest that Ni(I) can be stabilized.

isoltaed Ni(I) species with axial symmetry (g||=2.49, g^ =2.11) in NiH-SAPO-11 through thermal or hydrogen reduction

the g values are not sufficiently discriminatory to find the location of nickel(I)

Ni-APSO-5: Ni(I) replaces framework phophorous according to nearest phosphorous coordination and distance

31P and 2D ESEM spectroscopy provides detailed information about coordinationand can therefore discriminate between ion-exchanged and synthesized Ni materials

NiH-SAPO-5: Ni(I) is located in the center of a hexagonal prism (site SI)

methanol or ethylene absorbed on NiH-SAPO-5 and Ni-APSO-5

Metal centres in (silico-)aluminophosphate microporous materials

example:

31P ESEEM, 2D ESEEM

Incorporation of Mn(II) into framework sitesin the aluminophosphate zeotype AlPO4-20Unlike lattices of typical zeolites, AlPO4-20lattice is neutral. Substitution with transitionmetal provides ion-exchange ability or acidity

cw EPR: presence of a single Mn(II) site with a 55Mn hyperfine coupling of 8.7mT

X-band

W-band

Mn(II)

MnAlPO4-20 possess specific catalyticactivity in hydrocarbon cracking

D. Arieli, D.E.W. Vaughan, K.G. Strohmaier, D. Goldfarb, J. Am. Chem. Soc. 121 (1999) 6028-6032.

Incorporation of Mn(II) into framework sitesin the aluminophosphate zeotype AlPO4-20

X band W band

ESEEM: detection of weak superhyperfine coupling

ENDOR: detection of strong superhyperfine coupling

31P doublet: strong hyperfine interarction (3.14 Å)27Al no doublet: weaker hyperfine interaction (4.44 Å)1H, 14N, 27Al (5.44 Å), 31P


Probe host-guest interactions at the molecular levelin zeolite and molecular sieve materials

Incorporation of organic copper complexes (pyridine, ethylenediamine):

1. ZSM-5, NaYCu(II) exchanged zeolites

2. MCM-4large-pore molecular sieve materials for adsorptive

and catalytic applications involving large molecules

Cu(en)2 Cu(py)2

ZSM-5

NaY

MCM-41

solution

MCM-41

Cu(py)42+ complexes

H-Cu(II) distance: 3.05 Å

2D HYSCORE

W. Böhlmann, A. Pöppl, D. Michel, Colloids and Surfaces 158 (1999) 235-240.

In situ EPRspectroscopic measurements of catalysts under working conditions

Operando EPRin situ EPR in combination with other spectroscopic analysis andsimultaneous on-line product analysis (e.g. EPR/UV-vis/laser-Raman)

EPR at low temperature (4-300K) are generally preferred toenhance signal and prolong relaxation time

reasonable signal intensities can still be observed at high temperatures for certain transition metal ions and temperature up to 773 K are feasible(e.g. V4+, VO2+, Fe3+).

B.M. Weckhuysen, Chem. Commun. 97 (2002)

A. Brückner, E. Kondratenko, Catalysis Today 113 (2006) 16-24

A. Brückner, Topics in Catalysis 38 (2006) 1-3.

Example: operando studies of transition metal oxide catalysts, e.g. of supported VOx and CrOx catalysts in oxidative and non-oxidative dehydrogenation of propane.

… while EPR detects sensitively paramagnetic transition metal ions such as V4+, Cr5+ and Cr3+, UV-vis isa powerful monitor for diamagnetic TMI such as V5+ and Cr6+

since the latter give rise to intense charge-transfer transitions

In situ EPR on vanadium oxide based catalysts at 673KMotivation:

catalysts based on vanadium oxide belong to the most important active phase for heterogeneously catalyzed reactions involving surface reduction and reoxidation cycles

vanadium oxide is not applied as pure phase but in form of mixedbulk oxides such as vandium phospates (VPO) or highly dispersedon support materials such as TiO2

of great interest is the role of crystalline and amorphous phases in the catalytic process


1. capable of identifying the structural environment of isolated VO2+ species in supported catalysts

2. characterizing VO2+ sites in bulk oxides (so far only possible with EPR)3. in situ observation under reaction condition

EPR exchange line narrowing: sensitive monitor for disorder-related sensitivity

(NH4)2(VO)3(P2O7)2 (VPO catalysts)during ammoxidation of toluene:amorphous VO2+ containing phases are active components

EPR line shape analysis<B4>/<B2>2 characterizesexchange coupling betweenneighboring VO2+

: increases with exchange interaction

pure crystalline phase

amorphous phase

perturbation of spin-spin exchange upon contact with reactantincreases with exchange interaction

certain degree of structural disorder correlates with catalytic activity


EPR in heterogeneous catalysis

• CW EPR spectroscopy

– g, A tensor analysis, spectral fitting: oxidation state, intermediates, hint about

structure

• ESEEM, ENDOR and 2D HYSCORE spectroscopy– direct measurement of hyperfine coupling tensor terms: structure information

• EPR Line shape analysis– crystalline vs. amorphous phase

• Defects and radical processes on oxide surfaces

• Metal centers in microporous materials

• In situ EPR (“operando” EPR)

TOOL

APPLICATIONS

EPR literature used in this presentationBooks & Reviews:

J. Weil, J.R. Bolton, J.E. Wertz, “Electron paramagnetic resonance”, Wiley Interscience, New York, 1994.

J. R. Pilbrow, “Transition ion electron paramagnetic resonance”, Clarendon Press, Oxford, 1990.

A. Schweiger, “Pulsed Electron Spin Resonance spectroscopy”, Angew. Chem. Int. Ed. Engl. 30 (1991) 265-

292.

Journal Papers:

D. M. Murphy, C.C. Rowlands, Current Opinion in Solid State and Materials Science 5 (2001) 97-104.

E. Giamello, M.C. Paganini, M. Chiesa, D.M. Murphy, J. Phys. Chem. B 104 (2000) 1887-90

Y. Kohno, T. Tanaka, T. Funabiki, S. Yoshida, Phys. Chem. Chem. Phys. 2 (2000) 2635-2639.


P. Bera et al, Chem. Mater. 14 (2002) 3591-3601.

P.J. Carl, D.E.W. Vaughan, D. Goldfarb, J. Phys. Chem. B 106 (2002) 5428-5437.

M. Hartmann, L. Kevan, Chem Rev. 99(3) (1999) 635-663.

W. Böhlmann, A. Pöppl, D. Michel, Colloids and Surfaces 158 (1999) 235-240.


Z.Qu, L. Giurgiu, E. Roduner, Chem. Commun. (2006) 2507-2509.


B.M. Weckhuysen, Chem. Commun. 97 (2002)

A. Brückner, E. Kondratenko, Catalysis Today 113 (2006) 16-24

Documents

I. Introduction to EPR II. EPR in heterogeneous catalysis